In issue #163 we covered five of the most interesting bikes of this year’s North American Handbuilt Show. Hank Matheson of Bicycle Fabrication had this downhill prototype on display. It caught our eye because it stood in such stark contrast to the rows of glossy, finished frames. That, and because it appeared that a significant portion of the suspension relied on old bicycle inner-tubes and electrical tape…

The tube and tape rat’s nest was concealing a new suspension design. “It’s hidden because Jon Heim [was] actually writing the patent for that particular system,” said Matheson in our NAHBS video interview. Our educated guess was that the suspension used an auxiliary shock—perhaps something similar to Kona’s Magic Link, but rather than controlling geometry and suspension travel the shock controled the fore-aft movement of the rear axle.

Now that all the paperwork has been filed and a patent is pending (so reads the Sharpie writing on the frame), Matheson was willing to give us an exclusive first look at this new suspension design.

Unwrapped

Matheson worked in concert with engineer Jon Heim to create a suspension design that would allow the wheel to travel in a vertical arc, as well to travel rearwards when the suspension encounters a big hit.

Dubbed “2-D” as in two degrees of freedom. The main shock, in this case a Fox DHX RC 4, controls the majority of the suspension movement through the single pivot suspension’s 225mm of vertical travel. A polyurethane elastomer spring, the black wedge bolted to the down tube and to an eccentric pivot housed in the downtube, controls the suspension’s 19mm of fore-aft movement.

The coil main spring and elastomer auxillary spring work independent of each other. “You can’t measure the travel in vertical inches—it’s best measured in square inches,” said Matheson.

Background

Matheson and co-developer Heim set out to design a suspension that would better handle large square-edge impacts, which have a tendency to slow riders down, or stop them in the tracks. “Everyone’s had this experience riding a full suspension: you come up to a square-edge hit and it yanks the bike out from under you—it’s too big for the vertical travel to deal with. This suspension design allows the wheels to move back, up, and out of the way,” Matheson explained.

Matheson was inspired by the Cannondale Gemini DH, a decade-old downhill bike, developed for World Cup racers Cedric Gracia and Anne-Caroline Chausson that never made it past the prototype stages. “I liked what the rear axle was doing…Cannondale did a really good job of isolating one from the other.”

How it works

Where Cannondale used five pivots and an auxiliary air shock for the Gemini DH’s vertical and rearward travel Matheson looked to automotive suspension, which uses polyurethane bumpers to dampen forces coming into the chassis. While elastomers may be synonymous with the first generation of suspension forks and shocks, Matheson and Heim believe they are the right tools for the job.

Illustration by Jon Heim

“We’ve talked about a bunch of different ways of tuning the movement of the rear axle. We’re not asking for a ton of rearward travel, [an elastomer] is light, simple, and compact. If we went to an air shock, yes, it would be more tuneable on-the-fly, but it’s more than we need,” said Matheson.

The main suspension controls the majority of the suspension movement. The fore-aft suspension duties, handled by the elastomer and the eccentric pivot, only comes into play when the rear suspension encounters abrupt impacts in the 4:30 to 6:00 range of the rear wheel, as viewed from the drive side of the bike.

Once an impact is encountered, the elastomer allows for a controlled rearward movement of the rear wheel; the eccentric pivot allows the swingarm to rotate backwards and slightly upwards, away from the impact, while the main shock controls the suspensions vertical movement.

Because the eccentric allows the swingarm to rotate back and up, it does have an effect on the spring rate of the main shock. When activated, the movement of the eccentric pivot slightly steepens the shock angle, making the suspension more progressive.

No Switching

According to Matheson, the only thing these suspension designs have in common is the use of an eccentric pivot, in place of a linkage, and the eccentrics are used for two entirely different reasons.

Yeti uses an eccentric pivot to control and improve pedaling performance. The eccentric rotates backwards during the initial stroke, counteracting chain forces and increasing anti-squat to provide a firm pedaling platform. As the suspension compresses further, the eccentric rotates forwards, giving the suspension an active, plush feel.

In contrast, Bicycle Fabrications’ 2-D design uses an eccentric pivot to control the rearward movement of the axle. “My eccentric activates a separate shock and axle path, which the Yeti’s design does not,” says Matheson.

The next step

Now that a patent is pending Matheson plans to do a small run of 20-30 frames to get feedback from riders. After than he plans to do more tweaking and possibly increase production. If nothing else, this is a refreshing effort from a small company to improve mountain bike suspension.

Keep reading

There were a lot of cool mountain bikes at the 2012 North American Handmade Bicycle Show. See our coverage here.